This invention relates to cross-linked polyphenylene oxide compositions. More particularly, it relates to such compositions where polyphenylene oxide chains are cross-linked with amino ketone bonding.
The polyphenylene oxides are a body of thermoplastics disclosed in U.S. Pat. Nos. 3,306,874 and 3,306,875, incorporated herein by reference. They are characterized by a unique combination of chemical, physical and electrical properties over a temperature range of more than 600.degree. F., extending from a brittle point of minus 275.degree. F. to a heat distortion temperature of 375.degree. F. In particular, the polyphenylene oxides combine high tensile strength and tensile modulus with a high softening temperature, and excellent resistance to water, steam, strong acids and alkalies.
Some polyphenylene oxides also exhibit exceptional transport properties making them useful as membranes for separation processes such as ion exchange, ultra filtration, reverse osmosis, pervaporation and even gas permeation.
However, the polyphenylene oxides also have certain undesirable characteristics that are common to most thermoplastic materials. For example, their resistance to most common organic solvents is low. Aromatic and chlorinated hydrocarbon solvents dissolve phenylene oxide polymers, while other solvents and solvent vapors induce crazing in molded polyphenylene oxide parts under stress, causing loss of strength. The tensile properties of the resins decrease steadily with increasing temperature, the drop off sharply at about 200.degree. C. Further, under extreme prolonged stresss, molded parts formed from the polyphenylene oxides tend to creep, causing permanent deformation.
It is known that these disadvantages which are common to most thermoplastic materials, may be overcome by cross-linking the individual polymer molecules during, or after, the forming of the material into its final shape. Thus, if a sufficient number of cross-linking sites are present, the material can be cross-linked and will then no longer be soluble, but only swell to a greater or lesser extent. Also, while the phenomenon of solvent crazing is not fully understood, it appears to involve crystallization of the polymer molecules. As the mobility of the polymer molecule is limited by cross-linking, crystallization is no longer possible, and thus the problem of solvent crazing is removed. The limitation on molecular mobility also prevents the polymer from flowing, even above its melting point, thus preventing, to a large degree, creep and loss of tensile properties at increased temperature.
The polyphenylene oxides are, to a high degree, chemically inert, a desirable characteristic from a materials standpoint. However, because of this inertness the prior art has experienced difficulty in introducing cross-links between polymer chains, and structurally different units generally, by simple chemical processes. For example, prolonged heating in air will render the polymer insoluble in aromatic or chlorinated hydrocarbon solvents, but the degree of cross-linking accomplished is quite low, and the materials produced swell to a considerable degree.
Cross-linked polyphenylene oxides have been disclosed by Borman in U.S. Pat. No. 3,330,806 and by Schmukler in U.S. Pat. No. 3,406,147. Borman disclosed a cross-linkable polyphenylene oxide without the disadvantages of degradation and brittleness resulting from heat-induced cross-linking by introducing hydroxyl radicals into the polyphenylene oxide resin. The hydroxyl substituted polyphenylene oxide could then be cross-linked by reaction, for instance with a formaldehyde-releasing substance such as hexamethylenetetramine. Schmukler attempted to overcome deficiencies in cross-linked polyphenylene oxides by providing a plurality of side chain acyloxy groups on the polymer chain. Cross-linking could then be induced at elevated temperatures by aromatic substitution in the presence of a Lewis acid or by transesterification with a difunctional material reactive with the acyloxy group. A disadvantage of such cross-linked polyphenylene oxides as disclosed by Borman or Schmukler is that the cross-linked resin comprises by-products of the cross-linking reaction which are detrimental to the utility of such cross-linked resins for gas permeation purposes.
Ward et.al. in U.S. Pat. No. 3,780,496 disclose sulfonated polyxylelene oxide membranes for use in gas separations where the hydrogen ion form of the sulfonate substituent can be converted to a metal counter ion form. Ward et.al. disclose that such membranes have some utility in gas separation. A principal disadvantage is that the presence of water can be detrimental in membrane formation. Accordingly, the preparation of such membranes in a water-based coagulating system is impractical.